1. Field of the Invention
The present invention relates to a catalytic converter comprising: a cylindrical catalyst carrier having a catalyst carried therein, a retaining mat wound around an outer peripheral surface of the catalyst carrier, and a casing including a cylindrical carrier retaining portion for retaining the catalyst carrier in a state in which the retaining mat is compressed. The cylindrical carrier retaining portion is an assembly which includes diametrically split halves whose axially extending opposite-side edges are fitted over each other with fitted portions thereof bonded to each other.
2. Description of the Related Art
In a conventional type of catalytic converter, the cylindrical carrier retaining portion of the casing is formed to have a cross-section with a true circular shape.
In making a catalytic converter, the following processes are employed: (1) one of the halves is retained on a lower mounting member with its opposite-side edges turned upwards; (2) the catalyst carrier with the retaining unit wound around the carrier is placed between the opposite-side edges; (3) an upper mounting member retaining the other half with its opposite-side edges turned downwards is lowered; the catalyst carrier is clamped by both the halves with the retaining mat compressed, therebetween and the opposite-side edges of the other half are fitted over the opposite-side edges of the one half, (4) both the fitted portions are bonded to each other by a means such as welding.
In this case, the urging force provided in a direction toward the center of the catalyst carrier by the cylindrical carrier retaining portion is largest at a circumferentially bisecting position in each of the halves; is relatively sharply decreased from the bisecting position toward the fitted portions; and is approximately zero at the fitted portions.
Therefore, to make uniform the surface pressure on the catalyst carrier over an area from the circumferentially bisecting position in each of the halves to the fitted portions to increase the retaining force applied to the catalyst carrier by the cylindrical carrier retaining portion, it is required that the density of the retaining mat after compression assume a predetermined value at the circumferentially bisecting position in accordance with the urging force, and is increasingly varied from the circumferentially bisecting position to both the fitted portions in accordance with the decreasing variation of the urging force so as to provide compensation.
In the prior art, however, because each of the halves has an arcuate inner peripheral surface, the amount of opposite-side edges of each half biting into the outer peripheral surface of the retaining mat and the frictional force of the inner peripheral surface of each half to the outer peripheral surface of the retaining mat are greater in the course of clamping of the catalyst carrier by both the halves and hence, the retaining mat is excessively drawn in a circumferential direction.
As a result, the density of the retaining mat after being compressed is low at the circumferentially bisecting position in each of the halves and is extremely high in the vicinity of both the fitted portions.
Because of this, the non-uniform degree of the surface pressure on the catalyst carrier in the circumferential direction thereof (excluding areas corresponding to both the fitted portions and this will also be applied hereinafter) is increased in such a manner that the surface pressure is extremely low at the circumferentially bisecting position in each of the halves and is extremely high in the vicinity of both the fitted portions. This causes a reduction in retaining force applied to the catalyst carrier by the cylindrical carrier retaining portion.